An engine is usually equipped with a throttle apparatus (which is commonly referred to as a “throttle body”) for controlling the amount of air drawn into the engine. The typical throttle body includes a main passage equipped with a throttle valve for controlling the amount of air that flows through the throttle body. The throttle valve is closed at idling of an engine such that a minimal amount of air can be inducted into the engine.
In such an idle state of an engine, the amount of air inducted into the engine must be controlled in order to maintain a preferable idle state during changes in electrical and/or mechanical loads, such as operation of a compressor for air conditioning. Therefore, passage bypassing the throttle valve is provided and an idle speed actuator is installed in the bypass passage such that the amount of air bypassing the throttle valve can be controlled. An entrance and an exit of the bypass passage are located at either side of the throttle valve.
An actuator such as a levering apparatus actuates the throttle valve from outside of the throttle body. The bypass passage and the idle speed actuator are therefore also generally located at a position that does not produce interference with the actuator.
When the throttle valve is opened for acceleration, the air flowing through the narrow opening formed between the throttle valve and the interior wall of the main passage increases and passes the exit of the bypass passage at a high speed. Turbulence is generated by interference between the air-flow and the exit of the bypass passage. Consequently air-induction noise is produced at the exit and amplified by resonance in the bypass passage, which resultantly increases the total noise of the engine.